Telephone exchange system



Feb.. M9 wf s. B. WILLIAMS TELEPHONE EXCHANGE SYSTEM 2 Sheets-Sheet lFiled Oct. 24, 1934 ATTORNEY Feb., il, i936. s. B WILLIAMS SAE@TELEPHONE EXCHANGE SYSTEM' Filed Oct. 24, 1934 2 Sheets-Sheet 2 m mlF/Rsr 056005@ E I l 303 /NTERMED/ATE DECODER LAST DECODER /NVENTOR 5.5.W/L L /AMS ATTORNEY Patented Feb. 11, 1936 UNITED STATES PATENT OFFICEBell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 24, 1934,Serial No. 749,692

9 Claims.

This invention relates to telephone exchange systems and moreparticularly to systems in which connections between subscribers linesterminating in different offices of the exchange area are established byautomatic switching mechanism over groups of interoce trunks.

In exchange areas having a large number of local oflices it is noteconomical to provide direct groups of trunks for interconnecting eachoce with every other oice, particularly where the oices are widelyseparated and the traic load is heavy. In some exchange areas thereforeit is more economical to route *traino between certain orlices through atandem oiice which serves as a common intermediate distributing point.

It has heretofore been proposed to render more eiiicient the trunks in agroup directly interconnecting any two oiiices by limiting the number ofthe direct trunks to the maximum required for handling normal traiicbetween such oices and by automatically rerouting the abnormal oroverflow trafc between such oflices to a tandem oice where the traiiicmay be directly routed to the desired oiice. This proposal also enablesexisting direct trunk groups between oces to be employed without theaddition of further trunks thereto when traic loads increase beyond thecapacity of such trunk groups by rerouting calls through a -tandemoiice. To accomplish the rerouting of calls through a tandem oflice insystems employing central office senders which control the setting ofselector switches to establish connections, the sender must receive asignal when all trunks of a direct trunk group extending to an officeare busy in order that the sender may take the necessary steps toreroute the next connection for that trunk group over a different trunkgroup extending to the tandem oice which in turn has direct trunks alsoextending to the desired oince. This signal depends upon the closure ofa chain circuit controlled by the trunks of the direct group when alltrunks of the group become busy.

A sender may be given information to route a call to a trunk of a directtrunk group extending to a desired office and proceed to cause theselection oi a trunk in such group when there is only one idle trunk inthe group and since an interval of approximately 4.5 seconds may elapsebefore a selector switch controlled by the sender is actually positionedonthe terminals of the idle trunk, the trunk may during this intervalbecome seized in a previously initiated call and the later initiatedcall will not then be capable of completion as the selector switch wouldbe driven to the direct trunk group overflow position. To guard `againstthis condition, it is the object of this invention to signal the senderwhen there is still one available idle trunk and also when there is noidle trunk in a trunk group in order that rerouting to a tandem trunkgroup may be effected without the hazard of encountering a falseoverflow condition. It is a further object of the invention wheretraffic conditions are such as to warrant it to signal the sender whenthere are still two available idle trunks, again when there is only oneavailable idle trunk and again when there is no idle trunk in a trunkgroup in order that rerouting may be made at a time when it will be mostaccurately eiective.

In systems of the type disclosed in Patent 1,862,549 granted June 14,1932 to R. Raymond and W. J. Scully the establishment of connectionsfrorn an originating ofhce to any other cnice of the exchange area ismade possible by district and oice selectors which are set to select anidle trunk of the group of trunks extending to the ofice in which adesired subscribers line terminates by a sender common to allsubscribers lines of the originating oiice. A plurality of senders, anyof which may be taken for use by a calling line, is provided, eachsender having registers for registering both the ofce and numericaldesignations of a desired line dialed by a calling subscriber, andcontrolling apparatus for controlling the setting of a district and anoffice selector to extend a connection to the oce in which the desiredline terminates and for controlling other apparatus in the selected olcefor completing the connection. A plurality of decoders is also providedcommon to all of the senders, an idle one of which becomes associatedwith a sender taken for use by a calling subscriber. The decoder isprovided with ofce code registers to which the oice code is transferredfrom the registers of the associated sender and which is also providedwith as many route relays as there are trunking points to which callsmay be trunked from the originating office. The combined setting of theoice code registers of the decoder determines which one of these routerelays shall be operated for any particular call. The operated routerelay is then instrumental in establishing in the associated sender aregistration of the class of the call and a decoded oce coderegistration for directing the sender to control the setting of thedistrict and oice selector switches or the setting of the districtselector alone to select a group of trunks extending to the cnice inwhich the desired line terminates.

In accordance with one modication of the invention, each trunk groupextending directly from one ofiice to another oi'rice, to which it isdesirable to apply alternate routing through a tandem office, isprovided with a. chain circuit having two relays associated therewith,one `of which operates only when all but one of the trunks of the groupare busy and the other of which operates when all trunks of the groupare busy. Associated with each decoder are as many reroute relaysindividualized to each direct trunk group as there are direct trunkgroups to which the rerouting feature is to be app-lied.

When there is only one idle trunk in the` direct trunk group extendingto a desired oice, the chain circuit associated with such trunk group isso. established through contacts of operated relays of the trunks ofthat trunk gro-up as to cause the operation of the first of the relaysassociated with the chain circuit to signal all decoders that there isbut one idle trunk. When thereafter a decoder is taken for use by asender and the registration transferred thereto from the associatedsender indicates that a call is to be routed to this trunk group, theroute relay of the decoder which is instrumental in causing the routingof the calls to this trunk group will be operated and the sender will becaused to set selector switches to seize the remaining idle trunk of thetrunk group. As soon as the sender has received the translated code fromthe route relay of the decoder, the decoder is dismissed and then thereroute relay in each decoder allotted to the particular trunk group isoperated preparatory to rerouting the next call incoming to any decoderfor that particular trunk group to a tandem trunk group extending to atandem oice through which calls may be extended to the oiice in whichthe desired line terminates. As soon as the last idle trunk of thedirect trunk group has been seized by a selector switch, the chaincircuit is altered in such a manner that the second relay associatedtherewith signals all decoders that there are no more idle trunks in thedirect trunk group and maintains the reroute relays of all decodersappertaining to such trunk group operated. When thereafter any decoderis seized by a sender, with the reroute relay operated, the oice coderegisters of the decoder cause the operation of a tandem route relay forrouting the calls through a tandem oice to the oice indicated by theoffice code rather than the o eration of the direct route relay. Shouldone or more trunks of the direct trunk group thereafter become idle thereroute relay appertaining to that trunk group in each decoder is`released and the next subsequent call for that group of trunks willagain be routed thereto. Thus ythe relays associated with the chaincircuit of a trunk group are instrumental in counting one idle trunk andinforming all decoders when such condition exists. This insuressatisfactory operation of the rerouting feature of decoders when directtrunk groups do not have more than ten trunks and the proportion of thetraflic to be rerouted through a tandem oince will not exceed 20 percent.

Where direct trunk groups have more than ten trunks and the proportionof the traffic to be rerouted through a tandem oice exceeds 20 perce-nt, a more satisfactory operation of the rerouting feature ofdecoders may be made by employing a circuit such as disclosed in thesecond modification of the invention. In accordance with thismodication, each direct trunk group to which it is desirable to applyalternate routing is provided with a chain circuit having three relaysassociated therewith, one of which operates only when all but two of thetrunks of the group are busy, a second of which operates only when allbut o-ne of the trunks are busy, and a third of which operates when alltrunks are busy. This circuit is therefore enabled to count two idletrunks and to prepare all decoders sui'iciently in advance of the actualseizure of the last two idle trunks of the group for rerouting callsthrough a tandem oflice when there are no available idle trunks in thedirect trunk group to a desired oiiice.

For a clearer understanding of the invention reference may be had to thefollowing detailed description taken in connection with the accompanyingdrawings of which:

Fig. l shows schematically a calling subscribers line, a line nnder,district selector, oice selector, link circuit, sender and.decoder-connector of an originating oice; a subscribers line, incomingselectors and a nal selector of a distant oice; a group of direct trunksextending between the oices; and a tandem oilice indicated by arectangle through which connections may also be established between theoriginating and distant oices;

Fig. 2 shows a circuit for counting when all but one of the trunks ofthe direct group of Fig. l are busy and for counting when all trunks ofthe group are busy;

Fig. 2A shows a modication of the circuit of Fig. 2 for counting whenall but two, all but one and all trunks of a. ldirect group are busy;and

Fig. 3 shows such portions of one decodery accessible from thedecoder-connector of Fig. 1 as are necessary to an understanding of theinvention and other decoders schematically illustrated b-y the dot-dashrectangles also accessible from the decoder-connector.

The line finder district selector |02, oice selector |01, district nder|03 and sender selector |04 of the link circuit, sender |05 anddecoder-coni ector |06 being o1 the well-known type and functioning inthe usual manner have, for the purpose of simplifying the drawings, beendisclosed only schematically herein. For a complete disclosure anddescription thereof, reference may be had to Patent No. 1,862,549,granted to R. Raymond and W. J. Scully, hereinbefore referred to. Thedecoder 300, only such portions of which as are deemed necessary to anunderstanding of the present invention have been disclosed, is of thetype disclosed in the aforementioned patent. The tandem oilce equipmentindicated by the rectangle |08 of Fig. l is or" the type disclosed indetail in Patent No. 1,840,132, granted January 5, 1932 to T. H.Roberts. The incoming selectors |09 to ||2, inclusive and the nalselector H3 are also of the well-known type and therefore have been onlyschematically disclosed.

In order that the invention may be more clearly understood, it will beassumed that the subscriber at substation |00 of an originating oice ofan exchange area desires to establish a connection with a subscribersline ||4 which terminates at a distant oice of the same exchange areaand, upon removing his receiver from the switchhook, obtains aconnection with an idle sender in the originating oice through theoperation of the line finder |0|, district nder |03 and sender selector|04 in the well-known manner and then dials the once code letters andnumerical digits of the directory number of the wanted line |4.

acca-1e It willwfirst be assumed that atithe time decoder 300 is seizedby the decoder-connector |06,

In response to dialing, the office code and numerical designations areregistered in the sender in the well-known manner and following theregistration of the letters of the oiice code designation thedecoder-connector |06 is operated to associate an idle one or" aplurality of decoders with the sender. Three of the plurality ofdecoders are indicated by the rectangles 300, 30| and 302 of Fig. 3.Assuming that the first decoder 300 is at the time free, thedecoder-connector |06 causes the association thereof with the sender andoperates the decoder start relay 3|5.

Each decoder is provided with the usual receiving registers to which anassociated sender transfers the registered office code designations, aroute relay corresponding to each route over which connections may beextended from the originating' office and transmitting registers whichare selectively set by any operated route relay. The transmittingregisters are employed to transfer routing infomation to registers ofthe associated sender which in turn control the functioning of thesender to establish the desired connection over -district and officeselectors, or over a district selector only, by the route indicated bythe operated route relay of the decoder. Four of the route relays ofdecoder 300 are disclosed, of which relays 303 and 305 designate directroutes over which connections may be established from the originatingoffice directly to different distant or terminating oflices and relays304 and 300 designate tandem routes extending from the originatingoflice to different tandem offices. If it be assumed that there are twohundred local terminating ofces in the exchange area, there would betwo-hundred direct route relays such as 303 and 305 and as many tandemroute relays such as 304 and 300 as there are tandem oihces in the areato which the originating ofce has access.

In addition to the usual equipment the decoder is also provided with areroute relay corresponding to each direct route to which it isdesirable to apply alternative routing. Two of these reroute relays 301and 308 have been disclosed.

With the recoder idle, the operating winding of each reroute relay isnormally connected to the trunk group busy conductor of the group ofdirect interofce trunks forming a part of the corresponding directroute, over the back contact of a cut-'off relay. Nine groups of suchcut-off relays of four relays each may be provided, or thirty-sixrelays, over the back contacts of which the trunk group busy conductorsof as many as two hundred direct groups of trunks may normally extend tothe two hundred reroute relays, if it should be required thatalternative routing be applied to all direct routes. Two of thesecut-olf relays 309 and 3|0 have been disclosed. The nine groups ofcut-off relays are in turn controlled by a smaller group of three relays3| i, 3i? and 3|3 which are in turn controlled by relay 3|4 and thedecoder start relay 3|5. Thus, when the decoder is seized and the` startrelay 3|5 operates,

' relays 3|4, 3| I, 3|? and 3|3 and all cut-off relays,

'such as 309 and 3|0, operate in cascade to disconnect the operatingwindings of all reroute re'- lays of the decoder from the correspondingtrunk group busy conductors. An additional relay 3|6 is also providedwhich operates in parallel with relay 3|4 for providing locking groundsto the locking windings of all reroute relays. The start relay 3|5 alsofunctions in the usual manner to initiate the registration andtranslation functions ofthe decoder.

the direct route extending to the oflice in which the wanted line ||4terminates is available and that all but the last two trunks ||5 and IIBof the group of trunks included in this route are busy, as indicated bythe operated condition of their sleeve relays such as relay ill.Associated with each sleeve relay is a resistance, the resistancesassociated with relays IIB and H0 being disclosed at |20, |2| and |22,respectively. Each resistance has a normally open shunt eX- tending overcontacts of the sleeve relaywith which it is associated, and all of theresistances, when all of the trunks of the group are idle, are normallyconnected in a chain circuit extending from grounded batterytherethrough in series to conductor |23 and thence in series through thelower operating Winding of relay |24 and the winding of relay |25 toground. Relay |24 is a polarized relay, electrically biased by thecircuit through its upper winding, and so adjusted that it will operatein series wit-h one of the resistances, but will not operate or willrelease in series with two or more of the resistances. Relay |25 ismargined to operate only when all resistances have been removed from thechain circuit. It will thus be apparent that as each trunk of the groupis taken for use, one of the serially connected resistances will becomeshunted by the operation of the trunk sleeve relay and that when all butone of the trunks of the group have been seized, all but one of theresistances will become shunted and relayv Under the assumption thatwhen decoder 300 I is seized bo-th trunks ||5 and ||6 are idle and thatconsequently neither relay |24 nor relay |25 is operated, there will beno ground potential on the trunk group busy conductor |26 individual tothe group of trunks. of the decoder allocated to the route of which thisdirect trunk group is a part will therefore be unoperated. Upon theseizure of the decoder and the operation of start relay 3|5, the decoderthen proceeds to register the ofce code designations of the wanted linetransferred thereto from the office code registers of the sender and totranslate such office code by marking the code point all. This causesthe operation of relay 35S individual to the direct route of which thedirect trunks of Fig. 1 are a part and the operation of the properdirect route relay 305 in a circuit extending over the upper backcontact of reroute relay 308. With route relay 305 operated thetransmitting registers of the decoder are selectively set and the properrouting information is transmitted to the sender in the usual manner.Relay 3|5 also causes the operation of all Vcut-off relays such as 300and 3|0 thereby opening the operating circuits of all route relays sothat no reroute relay may be operated during the time that the decoderis making a routing thereby preventing the mutilation of the routingthat is at the time in progress.

The sender after receiving the required rout- The reroute relay 308 inginformation, dismisses the decoder by releasing the start relay 3|5thereby in turn releasing all of the cut-01T relays such as 309 and 3|0.Any reroute relay such as 301 or 308 may now be operated to change therouting on subsequent calls. The decoder may no-w be employed inconnection with another call. rIhe sender also proceeds to set thedistrict selector |02 and the office selector |01 to extend theconnection from the calling line to the proper trunk group extending tothe office of the wanted subscriber, and the office selector proceeds tohunt for an] idle trunk in the trunk group. It having been assumed thatall but the last two trunks of the group are busy, the office selectorwill hunt over the terminals of busy trunks and Will seize the firstidle trunk or trunk H5. The sender will thereupon proceed to control thesetting of the incoming selector in which the trunk ||5 terminates andthe nal selector I I3 to complete the connection to the wanted line H4.Following the completion of its controlling functions the sender isdismissed in the usual manner.

Upon the seizure of trunk ||5 its sleeve relay i IS is operated to shuntthe resistance |2 I. Since now all but one of the trunks of the groupare busy and therefore only one resistance |22 associated with thesleeve relay H9 of the only remaining free trunk lili, remains connectedin the previously traced circuit through the operating windings ofrelays |24 and |25, relay |24 operates, but relay |25 does not. It willnow be assumed that the decoder 300 is again seized due to theinitiatio-n of another call to the same distant oflice and that sincethe reroute relay 308 is still unoperated, there being no operatingcircuit therefor over trunk group conductor |25, the decoder in againmarking the same code point 3|? reoperates the relay 3|8 and the directroute relay 305. As before, the operation of route relay 305 causes thetransmission of translated routing information to the sender whereuponthe sender proceeds to extend a connection from the calling line to thedesired office over the one remaining idle trunk H5 of the trunk group.

Relay 3|8 upon operating establishes a circuit from battery over itscontacts, conductor |21, Winding of relay |28, back contact of relay |25to ground at the front contact of relay |24, relay |24 having operatedas an indication that there is but one available idle trunk in thegroup. Relay |28 upon operating locks directly toi battery independentof the control of relay 3|8 and connects ground to conductor |25. Theoperation cf relay |28 therefore makes a count of the fact that there isbut one remaining idle trunk in the associated trunk group. Until thedecoder has completed its functions and been dismissed by the associatedsender, the cut-off relays including relays 303 and 3|0 will remainoperated and therefore the connection of ground to ccnductor |26 willnot operate the reroute relay 308, thereby preventing the mutilation ofthe routing information at the time being established by the decoder.

When the last idle trunk in the group, for example, the trunk! i5 isseized, its sleeve relay |9 shunts the last unshunted resistance fromthe previously traced circuit through the windings of relays |24 and |25and relay |25 now operates. With both relays |24 and |25 operated, thelocking circuit of relay |28 is opened and ground is connected toconductor |26 independently of relay |28 over the front contacts ofrelays |24 and |25. In the meantime, the decoder having completed itsfunctions has been dismissed thereby releasing relays 3|5 and 3|8. Withrelay 3| 8 released relay |28 now releases. Relay 3|5 upon releasingreleases the operated cut-off relays such as 309 and 3|0 and, withground connected to conductor |26 over the front contacts of relays |24and |25, the reroute relay 308 is operated to transfer the code point3|] from the direct route relay 305 to the tandem route relay 306. Asimilar transfer is effected by the corresponding reroute relay in eachof the other decoders.

If, now, any of the decoders is seized on another call, as for exampledecoder 300, and the start relay 3|5 thereof is operated, relays 3| 4and 3|6 will be operated, relay 3|6 establishing locking circuits forall operated reroute relays of the decoder. Reroute relay 308 will,therefore, be locked operated and, assuming that the informationtransferred to the decoder from the associated sender signifies that theconnection desired is for a line terminating in the same distant oflice,the decoder will, because of the operated reroute relay 358, cause the.ope-ration of the tandem route relay 305 and the transmission ofrouting information to the sender whereby the sender will cause thesetting of the district selector and an office selector to select andidle trunk extending to the tandem oflice |08. The sender will thentransmit the necessary routing information to the sender of the tandemoffice whereby the connection may be further extended to the distantoffice. The connection is then completed in the distant oflice under thecontrol of the tandem sender, for example over the incoming selector 09and the final selector I3.

Should more than one .of the decoders be handling a call for the samedirect trunk group, for example, the trunk group disclosed in Fig. l,

at the time relay |24 thereof is operated, then as l of the rerouterelay of all decoders corresponding L to relay 300 to inform thedecoders that any subsequent call to that trunk group should bererouted. 'Ihese reroute relays will then remain operated under thecontrol of relays |24 and |25 of the trunk group and decoder startrelays :s

as long as all the trunks of the group are busy.

If one of the trunks becomes idle', relay |25 will release in turnreleasing the reroute relay, such as relay 308, of all busy decoders assoon as those decoders become idle. The circuits are now ready to countanother last call for the trunk group. If more than one trunk becomesidle relays |24 and |25 will both release and the reroute relays of thedecoders will release as previously described andthe circuitsconditioned to count when all but two of the trunks of the trunk groupbecome busy.

From the foregoing it will be apparent that the decoder and anyassociated sender are given information when there is but one availabletrunk in a direct trunk group and again when there is no idle trunk,suiciently in advance of the establishment of a possible call to thetrunk group, so that rerouting to a tandem route may be effected withoutthe hazard of losing a call.

This insures satisfactory operation of the rerouting feature of decoderswhen direct trunk groups do not have more than ten trunks and theproportion of the traiiic to be rerouted through a tandem oi'ce will notexceed 20 per cent Where direct trunk groups have more than ten trunksand the proportion of the traffic to be rerouted through a tandem officeexceeds 2O per cent, a more satisfactory operation of the reroutingfeature of decoders may be made by employing the modified countingcircuit disclosed in Fig. 2A, in lieu of the counting circuit of Fig. 2,conductors |33, |36 and |31 of Fig. 2 being joined to conductors |23,|23 and |21, respectively, of Fig. 1. Relays |34 and |40 are polarizedand electrically biased by the circuits through their upper windings andso adjusted that relay |34 will operate when two resistances areincluded in the chain circuit over conductors |23 and |33 but will notoperate or will release in series with three or more resistances andthat relay |40 will operate when one resistance is included in the chaincircuit but will not operate or will release in series with two or moreresistanccs. Relay |35 is margined to operate only when all resistanceshave been removed 'from the chain circuit. It will thus be 'apparentthat the operation of relay |34 is indicative that there are but twoidle trunks in 'the group, lthat the operation of relay |40 isindicative that there is but one idle trunk and that the operation ofrelay |35 is indicative that 'all trunks of the group are busy. i Whenall but two of the trunks of the group become busy and relay |34operates, an obvious circuit is establishedA for relay |4|. Should thedecoder 300 thereafter 'receive information for routing a call to thistrunk group and ground the code point 3|1, relay 3|8 of the decoder willoperate and remain operated until the decoder is dismissed and willclose the circuit from battery "at its contacts over conductors |21 and|31, upper back Contact of relay |42, inner upper Contact of relay v|4|,lowerv back contact of relay |42 to ground through the winding of relay|43. Relay |43 will operate and lock in a circuit from ground throughits winding and contacts, the winding of relay |42, the upper frontcontact of relay |4|,

to battery, but relay |42 will not operate in this locking circuit sinceits winding is shunted by battery supplied by relay 3|8 of the decoder.As soon, however, as the decoder is dismissed after having informed theassociated sender to route the call to the trunk group, relay 3|8releases permitting relay |42 to operate. Relay |42 upon operating opensat its lower contact the initial operating circuit of relay |43 and atits upper front contactA prepares the operating circuit for relay Whenthe last call which may be extended over Athe trunk group arrives in adecoder, for example the decoder 300, and the decoder again vgrounds thecode point 3H and reoperates relay 318, `the circuit for relay |38 iscompleted over conductors |21 and |31 and relay |38 operates lockingover its lower front contact, the lower contact of relay |4| to batteryat the back contact of relay |35. Relay |30 upon operating thus countsthe last call that may be extended to the trunk group and places groundon conductors |33 and |26 which is instrumental in operating rerouterelay 308 of the decoder as. soon as the decoder is dismissed followingits functioning to inform the associated 'sender'to'route a call to thelast available trunk of the'trunk group'. Thereafter, any subsequentcalls intended for this trunk group will be rerouted through a tandemoffice as previously described.Y

The arrival of the call to the trunk group at the time there were buttwo free trunks therein, causes the seizure of one of the remaining idletrunks and the consequent operation of relay |40 which, in turn, closesa shunt around the winding oi relay |43 extending from ground at thecontact of relay |40, through resistance |44, front contact and windingor relay |43 to ground. Relay M3 now releases but relay |42 remainsoperated. This is done so that should another trunk become idle before acall directed to the group has seized the last previously indicated idletrunk and relay |40 consequently releases, relay |42 will have noholding circuit and will release and open the previously tracedoperating circuit for relay |38. Relay |38 will not therefore operate aspreviously described and the decoder will thus be informed to route twocalls to the trunk group.

The arrival of the last call which the trunk group can handle, assumingthat all but one of the trunks are at the time busy, will busy the oneremaining idle trunk and thus cause the operation of marginal relay |35.Relay |35 upon operating releases relay |30 but maintains ground onconductors |36 and |26 to hold the reroute relay corresponding to relay308 operated. If thereafter only one trunk becomes idle, relay |38 willrelease, removing ground from conductor |26 to release the decoderreroute relay such as relay 308 of decoder 330, and to prepare thelocking circuit of relay |38 for counting another last call to the trunkgroup.

If two trunks l: ecome idle, both relays |40 and |35 will release, therelease of relay |40 permitting relay |42 to release, thus preparing thecircuit to count two possible vcalls for the trunk group. If more thantwo trunks become idle all of the relays |34, |35, |40, |4|, |42, |43and |38 will be released and will remain released so long as more thantwo trunks of the group remain idle.

What is claimed is:

l. In a telephone exchange system, a calling oiiice, called oriices, anintermediate ofce, switching mechanism located in said calling oice, agroup of direct paths extending from said switching mechanism to each ofsaid called ofces, a group of indirect paths extending from saidswitching mechanism through said intermediate office to each calledoflice, a register sender for recording the office code designations ofwanted vlines and for. controlling said switching mechanism to select apath to the called oice in which a wanted line terminates, translatingmeans associated with said sender for translating office coderegistrations to enable said sender to control said switching mechanism,reroute relays in said translating means corresponding to the groups ofdirect paths, each of said vreroute relays being operable to change thetranslation normally effective for routing a call to the correspondinggroup of direct paths to a translation effective for routing the call tosaid groupfof indirect paths, and means effective for operating thereroute relay corresponding to a particular group of direct paths whenall but one of that group of paths are actually busy and the translatingmeans has completed the direction of an initiated call to the last idlepath in that group.

2. In a telephone exchange system, a calling oiilce, called ofces, anintermediate oce, switching mechanism located in said calling ofiice, agroup of direct paths extending from said switchillg mechanism to eachof said called oices, a

group of indirect paths extending from said switching mechanism throughsaid intermediate ofce to each called oince, a register sender forrecording the oflice code designation of wanted lines and forcontrolling said switching mechanism to select a path to the calledoflice in which a wanted line terminates, a decoder associable with saidsender having route relays corresponding to all groups f paths extendingfrom said switching mechanism, operating circuits for said route relayscontrolled from said sender in accordance with the office codedesignation registered therein, reroute relays in said decodercorresponding to the groups of direct paths, each of said reroute relaysbeing operable to transfer the circuit normally eective for operatingthe route relay corresponding to a group of direct paths to a routerelay corresponding to said group of indirect paths, and means effectivefor operating the reroute relay corresponding to a particular group ofdirect paths when all but one of that group of paths are actually busyand the decoder has completed the direction of an initiated call to thelast idle path in that group.

3. In a telephone exchange system, a calling oiice, a called oice, anintermediate omce, switching mechanism located in said calling office, agroup of direct paths extending from said switching mechanism to saidcalled oliice, a group of indirect paths extending from said switchingmechanism through said intermediate oiiice to -said called oice, aregister sender for normally controlling said switching mechanism inresponse to the dialing of the ofce code of said called oiiice to routea connection to said called oiice over an idle one of said direct paths,a chain circuit for said group of direct paths including as manyresistances as there are paths in the group, a relay associated witheach direct path and operable upon the seizure of that path by saidswitching mechanism to short-circuit one of said resistances, a relay insaid circuit margined to operate only when all but one of the paths havebeen seized and all but one of said resistances have becomeshort-circuited, and means associated with said sender controlled bysaid marginal relay for causing said sender to reroute all callsdestined for completion to said called oiiice over said group of directpaths by way of said group of indirect paths only after said sender hascompleted the rout-ing of a call to the last idle one of said group ofdirect paths.

4. In a telephone exchange system, a calling oillce, a called oflice, anintermediate oilice, switching mechanism located in said calling ofce, agroup of direct paths extending from said switching mechanism to saidcalled olice, a group of indirect paths extending from said switchingmechanism through said intermediate olce to said called cnice, aregister sender for recording the oiice codes designation of a wantedline terminating in said called ofce and for controlling said switchingmechanism to select an idle path to said called'oice, vtranslating meansassociated with said sender for translating said oflice coderegistration to enable said sender to control said switching mechanism,a reroute relay in said translating means allocated to said group ofsaid paths, said reroute relay being operable to change the translationnormally effective for routing a call to the group of direct paths to atranslation eiective for routing the call to said group of indirectpaths, a group relay operable when all butlone of said direct paths isbusy, a code point relay in said translating vmeans operable wheneversaid translating meansis functioning to cause a call to be routed tosaid group of direct paths and an operating circuit for said rerouterelay controlled jointly by said group relay and said code point relayand effective only following the complete functioning of saidtranslating means to route a call to the last idle one of said group ofdirect paths.

5. In a telephone exchange system, a calling office, a called oflice, anintermediate office, switching mechanism located in said calling office,a group of direct paths extending from said switching mechanism to saidcalled omce, a group of indirect paths extending from said switchingmechanism through said intermediate oice to said called oflice, aregister sender for recording the oice code designation of a wanted lineterminating in said called oiiice and for controlling said switchingmechanism to select an idle path to said called oi'lice, translatingmeans associated with said sender for translating said office coderegistration to enable said sender to control said switching mechanism,a reroute relay in said translating means allocated to said group ofdirect paths, said reroute relay being operable to change thetranslation normally effective for routing a call to the group of directpaths to a translation eifective for routing the call to said group ofindirect paths, a first group relay operable when all but one of saiddirect paths is busy, a second group relay operable when all of saiddirect paths are busy, a code point relay in said translating meansoperable whenever said translating means is functioning to cause a callto be routed to said group of direct paths, a third grouprelay operableunder the joint control of said first group relay and said code pointrelay and maintained operated until said second group relay operates, anoperating circuit for said reroute relay controlled by said third grouprelay and effective only following the complete functioning of saidtranslating means to route a call to the last idle one of said group ofdirect paths, and a holding circuit for said reroute relay establishedby the operation of said second group relay.

6. In a telephone exchange system, a calling oflice, a called oiiice, agroup of trunks extend- .ing between said olices, switching mechanism insaid calling office for extending connections over said trunks to saidcalled oflice, a chain circuit for said group of trunks including asmany serially connected resistances as there are trunks in the group, arelay associated with each group and operable upon the seizure of saidtrunk to short-circuit one of said resistances, a first relay in saidcircuit margined to operate only when all but two of the trunks of saidgroup have been seized, and all but two of said resistances have becomesho-rt-circuited, and a second relay in said circuit margined to operateonly when all but one of said trunks have been seized and all but one ofsaid resistances have become short-circuited.

'7. In a telephone exchange system, a calling ofi-ice, a called oflice,an intermediate oflice, switching mechanism located in said callingofce, a group of direct paths extending from said switching mechanism tosaid called oce, a group of indirect paths extending from said switchingmechanism through said intermediate onice to said called oiice, aregister sender for normally controlling said switching mechanism inresponse to the dialing of the office code of said called of- -ce toroute a connection to said called ofce over an idle one of said directpaths, a chain circuit for said group of direct paths including as manyresistances as there are paths in the group, a relay associated witheach direct path and operable upon the seizure of that path by saidswitching mechanism to short-circuit one of said resistances, a firstrelay in said circuit margined to operate only when all but two of thepaths have been seized and all but two of said resistances have becomeshort-circuited, a second relay in said circuit margined to operate onlywhen all but one of the paths have been seized and all but one of saidresistances have become short-circuited, and means associated with saidsender and controlled by the operation of said rst and second relays forcausing said sender to reroute all calls destined for completion throughsaid called cnice over said group of direct paths by way of said groupof indirect paths only after said sender has completed the routing of acall to the last idle one of said groups of direct paths.

8. In a telephone exchange system, a calling oice, a called office, anintermediate oice, switching mechanism located in said calling office, agroup of direct paths extending from said switching mechanism to saidcalled oice, a group of indirect paths extending from said switchingmechanism through said intermediate ofce to said called oice, a registersender for recording the oiiice code designation of a wanted lineterminating in said called ofce and for controlling said switchingmechanism to select an idle path to said called cnice, translating meansassociated with said sender for translating said oiiice coderegistration to enable said sender to control said switching mechanism,a reroute relay in said translating means allocated to said group ofdirect paths, said reroute relay being operable to change thetranslation normally effective for routing a call to the group of directpaths to a translation eiective for routing the call to said group ofindirect paths, a rst group relay operable when all but two of saiddirect paths are busy, a code point relay in said translating meansoperable whenever said translating means is functioning to cause a callto be routed to said group of direct paths, a second relay operableunder the control of said rst group relay and said code point relay assoon as said translating means has completely functioned to route a callto one of the two` remaining idle paths of said group of direct paths, athird relay operable jointly under the control of said second relay andsaid code point relay, and an operating circuit for said reroute relaycontrolled by said third relay effective only following the completefunctioning of said translating means to route a call to the last idleone of said group of direct paths.

9. In a telephone exchange system, a calling cnice, a called office,switching mechanism located in said calling cnice, a group of preferredtrunks accessible to said switching mechanism for extending connectionsfrom said calling oice to said called office, a group of alternatetrunks accessible to said switching mechanism and to the switchingmechanism of other calling oices for extending connections from saidcalling ofces tosaid called omce and to other called ofces, a registersender for controlling said switching mechanism, and means for adjustingthe l selection control within said sender to cause said switchingmechanism to route one call for said called olice to said preferredgroup of trunks when all but one of said preferred group of trunks arebusy and to route succeeding calls for said called ofce to the alternategroup of trunks as long as all but one of said preferred trunks are busyeven though the last trunk has not been seized by switching mechanismand made busy by said one call.

SAMUEL B. WILLIAMS.

